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  • H01L29/66068—Multistep manufacturing processes of devices having a semiconductor body comprising crystalline silicon carbide the devices being controllable only by the electric current supplied or the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched, e.g. three-terminal devices
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  • H01L29/66409—Unipolar field-effect transistors
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  • H01L29/70—Bipolar devices
  • H01L29/72—Transistor-type devices, i.e. able to continuously respond to applied control signals
  • H01L29/739—Transistor-type devices, i.e. able to continuously respond to applied control signals controlled by field-effect, e.g. bipolar static induction transistors [BSIT]
  • H01L29/7393—Insulated gate bipolar mode transistors, i.e. IGBT; IGT; COMFET
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  • H01L29/78—Field effect transistors with field effect produced by an insulated gate
  • H01L29/7801—DMOS transistors, i.e. MISFETs with a channel accommodating body or base region adjoining a drain drift region
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  • H01L29/7801—DMOS transistors, i.e. MISFETs with a channel accommodating body or base region adjoining a drain drift region
  • H01L29/7802—Vertical DMOS transistors, i.e. VDMOS transistors
  • H01L29/7813—Vertical DMOS transistors, i.e. VDMOS transistors with trench gate electrode, e.g. UMOS transistors
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  • H01L29/76—Unipolar devices, e.g. field effect transistors
  • H01L29/772—Field effect transistors
  • H01L29/80—Field effect transistors with field effect produced by a PN or other rectifying junction gate, i.e. potential-jump barrier
  • H01L29/808—Field effect transistors with field effect produced by a PN or other rectifying junction gate, i.e. potential-jump barrier with a PN junction gate, e.g. PN homojunction gate
  • H01L29/8083—Vertical transistors

Definitions

• the subject matter disclosed herein relates to semiconductors and, more specifically, to silicon carbide (SiC) devices.
• SiC silicon carbide
• silicon carbide (SiC) based devices e.g., transistors such as metal-oxide- semiconductor field-effect transistors (MOSFETs) insulated gate bipolar transistors (IGBTs) or the like
• MOSFETs metal-oxide- semiconductor field-effect transistors
• IGBTs insulated gate bipolar transistors
• such devices typically employ short channels, tight cell pitch and include heavily doped (e.g., a sheet doping density (concentration) greater than about 2.5xl0 14 cm “2 , or in some embodiments, for example where a box profile depth is about 0.25 um, a doping concentration greater than about lxlO 19 cm “3 ) source regions to obtain a low on-state resistance (on-resi stance), Rds(on).
• SiC based devices often exhibit up to approximately twenty times nominal current density before saturation occurs, often exhibiting a much softer "quasi" saturation of drain family I-V characteristics.
• a silicon carbide (SiC) device may include a gate electrode disposed above a SiC semiconductor layer, wherein the SiC semiconductor layer comprises, a drift region having a first conductivity type, a well region disposed adjacent to the drift region, wherein the well region has a second conductivity type; and a source region having the first conductivity type disposed adjacent to the well region, wherein the source region comprises a source contact region and a pinch region, wherein the pinch region is disposed partially below the gate electrode, wherein a sheet doping density in the pinch region is less than 2.5xl0 14 cm "2 , and wherein the pinch region is configured to deplete at a current density greater than a nominal current density of the SiC device to increase the resistance of the source region.
• a silicon carbide (SiC) device may include a plurality of semiconductor device cells, wherein each of the plurality of semiconductor device cells comprises, a drift region having a first conductivity type, a gate electrode disposed above the drift region, a well region disposed adjacent to the drift region, wherein the well region has a second conductivity type, and a source region having the first conductivity type disposed adjacent to the well region, wherein the source region comprises a source contact region and a pinch region, wherein the pinch region is not completely disposed below the gate electrode, wherein a sheet doping density in the pinch region is less than 2.5xl0 14 cm "2 and a sheet doping density in the source contact region is at least 2.5xl0 14 cm “2 , and wherein the well region is doped about two times (2x) to about fifteen times (15x) greater than the pinch region.
• a method of fabricating a silicon carbide (SiC) semiconductor layer may include forming a source region having a first conductivity type, comprising implanting a pinch region of the source region to a sheet doping density of substantially less than 2.5xl0 14 cm “2 and implanting a source contact region of the source region adjacent to the pinch region to a sheet doping density higher than that of the pinch region, forming a well region having a second conductivity type adjacent to the source region by implanting the well region to a sheet doping density that is about two times (2x) to about fifteen times (15x) higher than that of the pinch region, and forming a gate electrode above a portion of the pinch region and a portion of the well region, wherein the gate electrode does not completely cover the pinch region of the source region.
• FIG. 1 is a schematic of a typical planar MOSFET device
• FIG. 2 is a schematic illustrating resistances for various regions of the typical MOSFET device of FIG. 1;
• FIG. 3 is a schematic of a portion of a SiC MOSFET device including a lightly doped pinch region, in accordance with embodiments of the present technique
• FIG. 4 is a graph showing the voltage and current waveforms illustrating short-circuit withstand time (SCWT) of 1.2 kilovolt (kV) rated SiC MOSFET devices under high current, wherein the cell pitches and doping vary between designs, in accordance with embodiments of the present techniques;
• SCWT short-circuit withstand time
• FIG. 5 is a graph illustrating output current-voltage characteristics of a 1.2 kV rated SiC MOSFET device at 25 °C for various gate-source voltages, wherein the pinch region is lightly doped and the cell pitch is reduced, in accordance with embodiments of the present techniques;
• FIG. 6 is a graph illustrating output current-voltage characteristics of a 1.2 kV rated SiC MOSFET device at 25 °C for various gate-source voltages, wherein the source region is heavily doped but the cell pitch is increased, in accordance with embodiments of the present techniques;
• FIG.7 is a graph illustrating output current-voltage characteristics of a 1.2 kV rated SiC MOSFET device at 150 °C for various gate source voltages, wherein the pinch region is lightly doped and the cell pitch is reduced, in accordance with embodiments of the present techniques;
• [uuiaj ⁇ i . 8 is a graph illustrating output current and voltage characteristics of a 1.2 kV SiC MOSFET device at 150 °C for various gate source voltages, wherein the source region is heavily doped and the cell pitch is increased, in accordance with embodiments of the present techniques;
• FIG. 9 is a graph illustrating output current and voltage characteristics of a 1.2 kV SiC MOSFET device at 25 °C, wherein the pinch region doping is varied between designs, in accordance with embodiments of the present technique;
• FIG. 10 is a graph illustrating output current and voltage characteristics of a 1.2 kV SiC MOSFET device at 150 °C, wherein the pinch region doping is varied between designs, in accordance with embodiments of the present technique;
• FIG. 11 is an enlarged portion of the graph of FIG. 9, in accordance with embodiments of the present technique.
• FIG. 12 is an enlarged portion of the graph of FIG. 10, in accordance with embodiments of the present technique.
• FIG. 13 is a graph illustrating tradeoffs between reducing saturation current density and increasing on-resistance using the embodiments disclosed herein versus other methods of reducing saturation current density.
• Present embodiments are directed toward semiconductor device designs (e.g., silicon carbide (SiC) based devices such as SiC MOSFET designs) in which the source region includes a depleteable pinch region (e.g., having a sheet doping density less than about 2.5xl0 14 cm “2 , or in some embodiments, between approximately 2.5xl0 12 cm “2 and approximately 5xl0 13 cm “2 ) that provides a nonlinear, current-density dependent resistance to improve current saturation characteristics under fault conditions.
• the disclosed pinch region generally functions to constrict or "pinch-off high source current densities, such as between approximately 4x and approximately lOx the nominal current density (JDS,nominai).
• the disclosed pinch region does not significantly increase on-state resistance of a device operating at nominal current densities, and therefore, does not significantly impact normal performance of the device. Accordingly, embodiments of the present disclosure may enable improving short-circuit fault ruggedness of semiconductor devices without compromising device performance during normal (i.e., non-faulting) operation. As such, the disclosed device designs are generally more adaptable for high power applications as compared to conventional device designs.
• Embodiments discussed herein relate to SiC based devices, for example, such as SiC based MOSFET devices. It should be appreciated, however, that the disclosed SiC device may be any type of field-effect transistor (FET) device. However, to simplify explanation, and to raciiitate aiscussion in the context of a concrete example, the present discussion will be provided in the context of the MOSFET described with respect to FIG. 1.
• FET field-effect transistor
• FIG. 1 illustrates an active cell of a planar n-channel field-effect transistor, for example a DMOSFET, hereinafter MOSFET device 10.
• MOSFET device 10 includes a semiconductor layer 2 (e.g., a silicon carbide (SiC) semiconductor layer) having a first surface 4 and a second surface 6.
• the semiconductor layer 2 includes a drift region 16 having a first conductivity type (e.g., an n-type drift layer 16), a well region 18 adjacent to the drift region and proximal to the first surface, the well region 18 having a second conductivity type (e.g., a p-well 18).
• the semiconductor layer 2 also includes a source region 20 that is adjacent to the well region 18, the source region having the first conductivity type (e.g., n-type source region 20).
• a gate insulating layer 24 is disposed on a portion of the first surface 4 of the semiconductor layer 2, and a gate electrode 26 is disposed on the gate insulating layer 24.
• the second surface 6 of the semiconductor layer 2 is a substrate layer 14, and the drain contact 12 is disposed on the bottom of device 10 along the substrate layer 14.
• Source/body contact 22 is disposed on top of the semiconductor layer 2, partially covering source region 20 and well/body regions 18.
• an appropriate gate voltage e.g., at or beyond a threshold voltage (VTH) of the MOSFET device 10
• VTH threshold voltage
• JFET junction field-effect transistor
• the channel region 28 may be generally defined as an upper portion of the well region 18 disposed below the gate electrode 26 and gate dielectric 24.
• the typical MOSFET device 10 illustrated in FIG. 1 includes a source region 20 that is heavily doped (e.g., a sheet doping density greater than about 2.5xl0 14 cm "2 , or in some embodiments, for example where a box profile depth is about 0.25 um, a doping concentration greater than about lxlO 19 cm "3 ) throughout to minimize on-state resistance of the device 10.
• FIG. 2 generally illustrates many of the same features illustrated in ⁇ ⁇ iG. 1.
• the contacts 22 of the MOSFET device 10 which generally provide an ohmic connection to the source electrode, are disposed over both a portion of the source region 20 and a portion of the well region or body region 18.
• the contact 22 is generally a metallic interface comprising one or more metal layers situated between these semiconductor portions of the MOSFET device 10 and the metallic source electrode.
• the portion of the source region 20 of the MOSFET device 10 disposed below the contact 22 may be referred to herein as a source contact region 42 of the MOSFET device 10.
• the portion of the well/body region 18 of the MOSFET device 10 that is disposed below the contact 22 may be referred to herein as a body contact region 44 of the MOSFET device 10.
• the various regions of the MOSFET device 10 may each have an associated resistance, and a total resistance when in an on-state (e.g., an on-state resistance or on-resi stance, Rds(on) of the MOSFET device 10.
• the on-state resistance or on-resi stance, Rds(on) may be represented as a sum of a plurality of resistances. For example, as illustrated in FIG.
• Rds(on) of the n-channel MOSFET device 10 may be approximated as a sum of: a source resistance (R s ) 30 (e.g., a resistance of n+ region 20 and a resistance of the contact 22); a channel resistance (Rch) 32 (e.g., a resistance of inversion channel region 28 illustrated in FIG.
• an accumulation resistance (Race) 34 e.g., a resistance of an accumulation layer between the gate oxide 24 and portion of drift layer 16 located between p-well regions 18
• a JFET resistance (RJFET) 36 e.g., resistance of undepleted neck region between p-well regions 18
• a drift layer resistance (Rdrift) 38 e.g., the resistance about the drift layer 16
• a substrate resistance (Rsub) 40 e.g., the resistance about the substrate layer 14.
• SiC based devices e.g., SiC MOSFET based devices
• present approach may be applicable to other types of material systems (e.g., silicon (Si), germanium (Ge), aluminum nitride (A1N), gallium nitride (GaN), gallium arsenide (GaAs), diamond (C), or any other suitable semiconductor) as well as other types of device structures (e.g., UMOSFET, VMOSFETs, insulated gate bipolar transistors (IGBT), insulated base MOS- controlled thyristors (IBMCT), junction field effect transistors (JFET), and metal-semiconductor field effect transistor (MESFET), or any other suitable device) utilizing both n- and p-channel designs.
• IGBT insulated gate bipolar transistors
• IBMCT insulated base MOS- controlled thyristors
• JFET junction field effect transistors
• MESFET metal-semiconductor field effect transistor
• Saturation current density may refer to the current at which the FET enters the saturation or "active" mode.
• the onset of saturation may refer to the drain current density where the differential output conductance has been reduced to one fifth of that of the linear region (e.g., defined by the inverse of the on-resi stance, where most power MOSFETs will operate in the on- state).
• u channel mobility
• W channel periphery
• Cox MOS gate-oxide- semiconductor capacitance
• L channel length
• VGS gate-source voltage
• VT threshold voltage.
• Lambda is a fitting parameter defining an assumed constant slope of IDS versus VDS after the onset of current saturation, and VDS is drain-source voltage.
• the negative temperature dependence of the threshold voltage may cause the saturation current density to increase further as the device may locally heat under fault conditions.
• the threshold voltage decreases, which may further increase the saturation current density and leads to high peak current during short-circuit fault conditions.
• the SiC MOSFET device 10 under the peak currents, the SiC MOSFET device 10 is able to function at temperatures, voltages, and/or currents outside desirable operatingranges.
• the time in which it takes the SiC MOSFET device 10 to fail under short-circuit conditions may be referred herein as the short-circuit withstand time (SCWT).
• SCWT short-circuit withstand time
• the SiC MOSFET device 10 may be beneficial to design the SiC MOSFET device 10 to extend the SCWT such that a scheme may be employed to handle the short-circuit fault in some desirable way (e.g., shut the device off) before the device or system is damaged or degraded. It may be appreciated that, in addition to limiting peak currents under fault conditions, present embodiments discussed below can also enable longer SCWT (e.g., between 5 and 20 microseconds ( ⁇ )) to give such fault management mechanisms sufficient time to turn off the device before the device is damaged or degraded. In some embodiments, the SCWT may be extended when supporting greater than 30 percent of rated drain-source voltage.
• a SiC device 60 e.g., a SiC MOSFET device having a source region 20 that includes a lightly doped pinch region 43 is shown in the schematic of FIG. 3.
• the SiC device 60 may include a number of semiconductor device cells and each of the semiconductor device cells may include at least the source region 20 including the pinch region 43 and the source contact region 42, the well region 18, and the drift region 16.
• the pinch region 43 is disposed adjacent to the source contact region 42 and the well region 18. It should be understood that the light doping of the pinch region 43 enables depletion of or "pinching off free carriers between the source region 20 and the channel region 28 under high current flow at on-state.
• the source contact region portion 42 which is doped significantly more than the pinch region 43, may have a sheet doping density greater than approximately 2.5x10 14 cm “2 , or in some embodiments between approximately 2.5xl0 14 cm “2 and approximately 5xl0 15 cm “2 or in some embodiments between approximately 1.8xl0 15 cm “2 or approximately 3.75xl0 15 cm “2 ).
• the sheet doping density in the pinch region 43 may be less than about 2.5xl0 14 cm “2 , or in some embodiments, between approximately 2.5xl0 12 cm “2 and approximately 5xl0 13 cm “2 .
• the width 61 of the pinch region 43 also affects the amount of source series resistance provided by the depleted pinch region 43 under high current flow.
• the length 61 of the lightly doped pinch region 43 may be selected based on desired source series resistance.
• the length 61 of the pinch region 43 may vary between approximately 0.5 micrometers ( ⁇ ) and approximately 2.5 ⁇ .
• the sheet doping density of the pinch region 43 times the length of the pinch region 43 is from approximately 1.25xl0 8 cm “1 to 125xl0 8 cm “1 .
• the channel region 28 of the SiC device 60 is in an on-state
• the well region 18 is doped by about two times (2x) to about fifteen times (15x) greater than the pinch region 43.
• the difference in the sheet doping density of the well region 18 and the pinch region 43 may enable backchannel depletion in the depletion region 45 during high current via the feedback mechanism discussed above.
• at least a portion of the pinch region 43 may be partially disposed below the gate electrode 26.
• the pinch region 43 is disposed below the gate electrode 26 and not below the gate electrode 26 (e.g., the pinch region 43 is not completely under the gate electrode 26).
• any portion of the pinch region 43 may be disposed below the gate electrode 26 that is sufficient to provide the benefits as described herein.
• substantially most of the pinch region 43 (as determined by the length 61) may not be disposed below the gate electrode 26.
• the gate region does not overlap the N+ source contact region 42 in active cell regions.
• Providing at least a portion of the pinch region 43 beneath the gate electrode 26 may provide better performance (e.g., limits saturation current density while keeping a lower on-state resistance under normal operating conditions) as compared to configurations where a portion of the pinch region 43 does not extend from, or is completely contained under, the gate electrode 26.
• One or more dimensions may be varied to adjust characteristics of the SiC device 60.
• an "overhead" dimension of the cell, or cell width not covered by gate (collectively referred to as "m") and/or a gate to source spacing (LGS) may be reduced to reduce channel 28 resistance or overall on resistance.
• the m and/or LGS dimensions may be "process technology" limited distances, that is, based on, or limited by, processing technology such as minimum feature size, layer to layer alignment tolerances, or the like.
• the LGS dimension may be used to provide the pinch region 43 that increases series resistance under higher than nominal current.
• the increase in series resistance enaoied by the pinch region 43 having a length that extends from under the gate electrode 26 may enable a reduced cell pitch (e.g., ⁇ 6.5um) in comparison to a length that does not extend from under the gate electrode 26 as it uses length to add source pinch resistance.
• a reduced cell pitch e.g., ⁇ 6.5um
• using a length of such a region that does not extend from under the gate electrode 26 (or is contained under the gate electrode) increases the gate-source overlap area for a given pinch resistance, which increases a gate to source capacitance C gs and slows the switching speed of the device.
• the presently disclosed embodiments that use the pinch region 43 having a length that extends from under the gate electrode 26 may enable a reduced cell pitch (lower channel resistance) and C gs and increased switching speed of the SiC device 60 for a given pinch resistance, as compared to using a length that does not extend from under the gate electrode 26.
• typical SiC based devices may include certain features that provide undesirable short-circuit performance.
• the inversion channel mobility may be low, which may lead to reducing cell pitch, to increase periphery, and to shorten the channel region 28 in order to improve on-resistance.
• cell pitch refers to the minimum repeat dimension from any point on the cross- sectional view of the MOSFET device 10 illustrated in FIG. 2.
• reducing cell pitch may lead to high saturation current densities.
• combining the above features with small chip sizes may lead to undesirable short-circuit performance of the devices.
• the SiC device 60 with the lightly doped pinch region 43 may enable reduced cell pitch and low on-resistance across nominal operating temperatures, while significantly improving short-circuit performance.
• FIG. 4 is a graph 46 showing short-circuit withstand time (SCWT) of 1.2 kilovolt (kV) SiC MOSFET devices under high current short-circuit conditions for different designs.
• Curve 47 represents a SiC device 60 of the present approach having a lightly doped pinch region 43 (e.g., sheet doping density of approximately 9xl0 12 cm "2 ) and a small cell pitch
• Curve 49 represents a SiC MOSFET having a heavily doped source region 20 (e.g., sheet doping density of greater than about 2.5xl0 14 cm “2 ) and a larger cell pitch (e.g., approximately 6 ⁇ to 6.5 ⁇ , approximately 1.27 times larger than the cell pitch of the design represented by curve 47).
• Curve 48 represents a SiC MOSFET having a heavily doped source region 20 and a larger cell pitch (e.g., approximately 9 ⁇ to 9.5 ⁇ , approximately 1.92 times larger than the cell pitch of the design represented by curve 47).
• the presently disclosed SiC device (SiC device 60 described above), represented by the curve 47, demonstrates an improved short-circuit performance (e.g., a SCWT of approximately 7.21 ⁇ ), compared to the SCWTs provided by other designs (e.g., 3.72 ⁇ and z. ⁇ ).
• the design represented by curve 47 demonstrates the lowest peak current and lowest current over most of the duration of the fault as compared to the other designs represented by curves 48 and 49.
• FIG. 5 is a current-voltage (IV) plot 50 for the embodiment of the SiC MOSFET 60 represented by the curve 47 in FIG. 4 operating at different gate biases (i.e., 10V, 12V, 14V, 16V, 18V, and 20V) at 25 °C.
• FIG. 6 is an IV plot 52 for the SiC MOSFET device represented by the curve 48 in FIG. 4 at different gate biases (i.e., 10V, 12V, 14V, 16V, 18V, and 20V) at 25 °C.
• the on-resistance is substantially the same between the two designs under nominal current ratings of approximately 20-3 OA. Therefore, in addition to providing superior short-circuit performance, as indicated by FIG. 4, the lightly doped pinch region 43 of the presently disclosed designs does not significantly increase the on-resistance of the device.
• FIG. 7 is a current-voltage (IV) plot 54 for the embodiment of the SiC MOSFET 60 represented by the curve 47 in FIG. 4 operating at different gate biases (i.e., 10V, 12V, 14V, 16V, 18V, and 20V) at 150 °C.
• FIG. 8 is an IV plot 56 for the SiC MOSFET device represented by the curve 48 in FIG. 4 at different gate biases (i.e., 10V, 12V, 14V, 16V, 18V, and 20V) at 150 °C.
• the on- resistance of the SiC MOSFET 60 is lower than the on-resistance of the other SiC MOSFET device at nominal operating currents.
• the curve 57 in graph 54 indicates drain current of 30 A at approximately 3 V
• the curve 58 in graph 56 shows drain current of 30 A at approximately 5V.
• the presently disclosed device design enables similar on- resistance as the other SiC MOSFET device at 25 °C, as indicated by FIGS. 5 and 6, and further enables lower on-resistance than the other SiC MOSFET device at 150 °C, as indicated by FIGS. 7 and 8, all while providing superior short-circuit performance, as indicated by FIG. 4.
• the drain-source voltage is represented on the x-axis in units of volts and the drain current density (saturation current density) is represented on the y-axis in units of amps per centimeter square (A/cm 2 ).
• the onset of saturation or saturation current density may refer to conductance being one fifth of the value at nominal current densities (e.g., approximately 200 A/cm 2 ).
• a graph 70 illustrates output current and voltage characteristics of a 1.2 kV SiC MOSFET device at 25 ° C where pinch region 43 doping is varied between designs.
• sheet doping density for the pinch region 43 may be varied between approximately 2.5xl0 12 cm “2 and approximately 2.5xl0 14 cm “2 .
• the graph 70 illustrates that the design with the lightest aopea pincn region 43 (e.g., 2xl0 12 cm “2 ) demonstrates the lowest saturation current density, smallest output conductance per unit area, and a substantially constant current versus drain voltage dependence in curve 72.
• curve 82 which represents the design with a conventional level of doping in the source region 20 (e.g., sheet doping density of about 2.5xl0 14 cm “2 ), includes a saturation current density of approximately greater than 2700 A/cm 2
• curve 74 which represents a design with a lower level of doping than curve 82 (e.g., sheet doping density of about 5xl0 12 cm “2 )
• curve 72 which represents the design with the lightest doped pinch region 43 (e.g., sheet doping density of about 2.5xl0 12 cm “2 )
• using the lightest doped pinch region 43 enables reducing the saturation current density to 600 A/cm 2 , which is an approximately 60 to 70 percent reduction from the next lowest saturation current density of approximately 1850 A/cm 2 demonstrated by the design with the higher level of doping in the pinch region 43, and an approximately 70 to 80 percent reduction from the saturation current density of approximately 2700 A/cm 2 demonstrated by the design with the standard level of doping in the source region 20.
• the design with the lightly doped pinch region 43 employs the feedback mechanism described above, which may result in the nearly constant saturation current density and linear drain voltage dependence. That is, the saturation current density of curve 72 may remain substantially the same as drain-source voltage increases.
• the saturation current density of the design with the conventional level of doping in the source region 20 does not show voltage hard saturation condition or high output conductance because the saturation current density continues to rise as the drain-source voltage rises. That is, the curve 82 has an increasing current continuing to rise after the onset of current saturation, whereas curve 72 shows a substantially more constant current density after reaching the onset of current saturation.
• the lower saturation current density of the design with the lightly doped pinch region 43 results from the free carrier region being "pinched off," as discussed above.
• peak current may be saturated at 5 to 10 times nominal current density (JDS,nominai)
• peak current of the other designs such as the design with the standard level of doping (e.g., Ixl0 19 cm “3 )
• the design with the lightly doped pinch region 43 may improve short-circuit fault condition performance because the saturation current density is substantially lower than the other designs (e.g., curves 74, 76, 78, 80, and 82).
• FIG. 10 is a graph 90 that illustrates additional output current and voltage characteristics of a 1.2 kilovolt SiC MOSFET device at 150° C, where pinch region doping is varied between designs.
• Graph 90 illustrates results for a subset of the embodiments depicted in FIG. 9, except that FIG. 10 shows the characteristics of these embodiments operating at a temperature 125 °C higher than in FIG. 9.
• the design that includes the lightly doped pinch region 43 (e.g., sheet doping density of about 2.5xl0 12 cm “2 ) produces the lowest saturation current density of approximately 1850 A/cm 2
• the design with the standard level of doping (e.g., sheet doping density of about 2.5xl0 14 cm “2 ) produces the highest saturation current density of approximately 3500 A/cm 2 .
• the saturation current densities increase when the temperature of the SiC MOSFET device 10 increases under fault conditions, as shown in graph 90.
• FIG. 11 is an enlarged portion of the graph 70 of FIG. 9.
• the enlarged portion of the graph 70 generally shows that the design with the lightest doped pinch region 43 does not demonstrate a substantial increase in on-state resistance (Rds(on)) at nominal current densities.
• the design with the lightly doped pinch region 43 as represented by curve 72, has a substantially similar slope as the designs with increasing levels of doping, represented by the curves 74, 76, 78, 80, and 82.
• the designs with different levels of doping show similar voltage drops because the curves 72, 74, 76, 78, 80, and 82 are bunched together.
• conduction losses that result from using a lightly doped pinch region 43 to reduce the saturation current density may be negligible in some embodiments.
• the on-state resistance of the device 60 may not substantially increase as a result of the lightly doped pinch region 43.
• FIG. 12 is an enlarged portion of the graph 90 of FIG. 10.
• the increase in on-resistance between the design with the lightly doped pinch region 43 (e.g., sheet doping density of about 2.5xl0 12 cm “2 ) (curve 72) and the design with the standard level of doping (e.g., sheet doping density of about 2.5xl0 14 cm “2 ) (curve 82) is not significantly large (less than 20 percent) during nominal current of 200 A/cm 2 .
• FIG. 13 is a graph 100 illustrating tradeoffs between reducing saturation current and increasing on-resistance using the embodiments disclosed herein versus other methods.
• Curve 102 represents an embodiment of SiC device 60 illustrated in FIG. 3, while the curve 104 represents SiC MOSFET devices that use other methods of adding source series resistance, connecting a series resistor externally to the SiC MOSFET device, or the like. As depicted, the curve 102 indicates a greater reduction in the saturation current density is achieved with a significantly smaller increase in on-resistance, when compared to devices represented by the curve 104. That is, reducing the doping of the pinch region 43 lowers the saturation current density from approximately 4750 A/cm 2 at an on-resistance of approximately 1 to approximately 2250 A/cm 2 at an on-resistance of approximately 1.18 in curve 102.
• curve 104 shows that the same decrease in saturation current density results in an increase in on-resistance from 1 to approximately 1.8, which is an approximately 50 to 55 percent increase in on-resistance relative to the on-resistance enabled by the disclosed pinch region 43 designs.
• the disclosed pinch region designs reduce the saturation current density from approximately 4750 A/cm 2 to approximately 2250 A/cm 2 (e.g., approximately 50 percent to 60 percent reduction in saturation current density), while increasing on-resistance from 1 to approximately 1.18 in curve 102, (e.g., approximately 5 percent to 50 percent increase in on-resistance).
• curve 104 shows that the same decrease in saturation current density results in an approximately 80 percent increase in on-resistance (e.g., from 1 to 1.8) using methods other than the disclosed pinch region 43.
• the feedback mechanism may enable non-linear source series resistance because the high current causes depletion of the pinch region 43. Depletion of the pinch region 43 increases resistance, which causes a forward drop in voltage to increase, which further depletes the pinch region 43 and increases resistance. Accordingly, non-linearity of the source series resistance provided by reducing the doping level of the pinch region 43 may enable reduced saturation current density while minimally increasing on-resistance under nominal operation.
• the inventors have provided an improved SiC device and method of fabricating thereof.
• technical effects of the disclosure include using a lightly doped pinch region to improve the saturation characteristics of semiconductor devices for power conversion applications (e.g., SiC MOSFET devices).
• the disclosed devices have a source region that includes a lightly doped pinch region that exhibits higher resistance under higher current density ⁇ e.g., high drain source bias) than that of nominal use conditions (e.g., 4x to lOx JD,nominai), which enhances the short-circuit fault condition ruggedness of the device without substantially increasing on-resistance during normal operation.
• Ranges disclosed herein are inclusive and combinable (e.g., ranges of “approximately 2.5xl0 12 cm “2 and approximately 2.5xl0 14 cm “2 ", is inclusive of the endpoints and all intermediate values of the ranges of “approximately 2.5xl0 12 cm “2 and approximately 2.5xl0 14 cm “2 “ etc.).
• “Combination” is inclusive of blends, mixtures, alloys, reaction products, and the like.
• first,” “second,” and the like, herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another, and the terms “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.
• the modifier “approximately” or “about” used in connection with a quantity is inclusive of the state value and has the meaning dictated by context, (e.g., includes the degree of error associated with measurement of the particular quantity) and may be used interchangeably.

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